Upon fabrication of a semiconductor device, an image is obtained by a lithography technique in which a resist is applied to the surface of a silicon wafer to form a photosensitive film, the film is exposed to light to form a latent image, and the latent image is then developed to form a negative or positive image.
With the advancement of high integration, high density assembly, miniaturization and speedup of semiconductors toward IC, LSI and further VLSI, demands on minute processing of elements are enhanced, and there is a demand for development of techniques forming a minute pattern having a line width of 0.5 .mu.m or less at present. It is however extremely difficult to form such a minute pattern by the conventional lithography technique making use of near ultraviolet radiation or visible radiation. Its yield is also lowered to a significant extent.
Therefore, in order to enhance the resolution of exposure, there are studied other lithography techniques making use of far ultraviolet radiation (short wavelength ultraviolet radiation), krypton fluoride laser (hereinafter referred to as "KrF excimer laser") beam or the like, which is shorter in wavelength than near ultraviolet radiation, instead of the conventional lithography technique utilizing near ultraviolet rays having a wavelength of 350-450 nm.
Novolak resins are widely used as base polymers in the conventional lithography technique utilizing near ultraviolet radiation. However, the novolak resins have been known to involve problems that sufficient sensitivity can not be achieved, and the form of the resulting pattern is deteriorated because they scarcely transmit rays having a short wavelength, such as far ultraviolet radiation and KrF excimer laser.
In recent years, therefore, attention has been attracted to a resist system in which a compound (i.e., a photo-induced acid generator) which generates an acid upon exposure to active rays (i.e., activated radiation) such as light or electron beam is combined with a compound which is decomposed by an acid catalyst to become soluble in a developer with a view toward achieving high sensitivity and high resolution. For example, acetal compounds (Japanese Patent Application Laid-Open No. 89003/1973), enol ether compounds (Japanese Patent Application Laid-Open No. 12995/1980) and the like have been proposed as acid-sensitive compounds used in combination with the photo-induced acid generator. Besides, a resist composition for minute processing, which is composed of a three component system of a base polymer, a photo-induced acid generator and a substance (an acid-sensitive compound) which reacts in the presence of an acid as a catalyst, which has been generated by the exposure to light, to change the solubility and the like of the base polymer, has been developed. It has been proposed to use, as the base polymer (a resin binder), a polymer having an orthoester group in its main chain (Japanese Patent Application Laid-Open No. 17345/1981), a polymer having a tert-butyl ester group of a carboxylic acid or a tert-butyl carbonate group of phenol (Japanese Patent Application Laid-Open No. 27660/1990), or the like. However, these methods are still insufficient from the viewpoints of sensitivity, storage stability, stability with time, resolution, pattern form and the like and hence fail to provide a fully satisfactory resist composition.
It has been proposed to use a hydrogenated polyvinylphenol derivative as a base polymer for the purpose of enhancing transmittance (Japanese Patent Application Laid-Open No. 249673/1993). According to this method, resolution is improved to some extent. In view of a recent higher standard of performance requirements on resists, however, further improvements are requested on resolution, pattern form and resistance to etching in particular.
As more recent examples, it has been proposed to bond a t-butoxycarbonyl group, t-amyloxycarbonyl group, t-butyl group, t-hexyl group, allyl group, 2-cyclohexyl group or the like as a protecting group, which is decomposed by an acid or base, to an alkali-soluble resin (Japanese Patent Application Laid-Open No. 158363/1992) or use a compound having a tert-amyl structure in its molecule (Japanese Patent Application Laid-Open No. 269754/1992) in order to attain higher sensitivity and heat stability. In view of a recent higher standard of performance requirements on resists, however, further improvements are requested on resolution, pattern form and resistance to etching in particular as described above.
It is more important that a resist film formed with any of the above-described resist compositions can not fully meet heat stability at a practical level and has involved a problem that a resist form is deteriorated by heat generated in an etching process upon transferring a resist pattern to a substrate (for example, silicon substrate), resulting in a failure to exactly transfer the resist form to the substrate.
In order to solve this problem, there has been proposed a resist composition in which a group capable of forming a secondary carbonium ion intermediate having a proton usable in a reaction upon cleavage is used as a protecting group to be decomposed by an acid or base in an alkali-soluble resin, thereby improving its heat stability (heat resistance) (Japanese Patent Publication No. 19139/1993). The resist film described in this document is recognized to have improved thermal properties compared with a resist having a t-butyl group as a group unstable to an acid. However, its sensitivity required of a resist film is markedly deteriorated. The resist composition is hence unpractical.
As described above, the sensitivity of resist compositions is improved to a considerable extent by investigating such a protecting group. In view of a recent higher standard of performance requirements on resists, however, further improvements are requested on resolution, heat resistance, form of resist pattern and resistance to etching in particular.